This invention is directed to YIG components in general and more specifically to a YIG component comprising a magnetic circuit for generating an homogeneous magnetic field in an air gap of the magnetic circuit, and at least one ferrite crystal arranged in said air gap and having a magnetic resonance frequency which may be controlled dependent on the strength of said homogeneous magnetic field.
"YIG components" is a generic term for devices using ferrite crystals, that is thin layers or crystals of YIG (yttrium-iron-garnet), LiF (lithium-ferrite) NiZnFe (nickel-zinc-ferrite), etc., as resonators in, for example, electric oscillators, filters and discriminators. YIG components are used in high frequency applications for frequencies from about 500 MHz and upwards. Electromagnetic frequencies in this range are often denoted "microwaves" and electric circuits operating at these frequencies are denoted "microwave circuits" hereinafter.
In order to be able to provide a resonator using a ferrite crystal, a strong, homogeneous magnetic field is required in which the ferrite crystal is arranged. The magnetic field is generated by a magnetic circuit comprising an electromagnet or a permanent magnet in combination with a magnetic iron structure. The magnetic resonance frequency of the resonator is directly proportional to the strength of the magnetic field. It follows from this that when using an electromagnet, the resonance frequency of a YIG component may be controlled electrically via the current through said electromagnet. The ferrite resonator has a number of good features and is characterized by a high Q-value and in that it can be controlled electrically within very broad frequency ranges (several octaves).
The majority of prior-art YIG components have a design in which the electromagnet completely or partly constitutes the housing and carrier for the remaining components, such as said ferrite crystal, microwave circuits etc., required to make up the intended YIG component. Because magnetic iron is a material which is difficult to work, the intention has been to provide an uncomplicated mechanical structure for the YIG component. An example of a prior-art YIG component is shown in JP 1-152804 (to Uehara). The YIG component comprises a magnetic circuit including an electromagnet having a magnetic core forming a box-shaped housing (13, 13a) of a two-part structure, the two parts being identical. Each part has a pole pin (16, 16a) surrounded by a coil (25, 25a). The pole pins define a pole gap in which a YIG element is positioned. The YIG element is included in a magnetostatic wave generator, which is fixed to a carrier (21, 20). The two parts have recesses which form rectangular holes in opposite sides of the housing. When the two parts are joined together to form the housing, the carrier (21, 20) is positioned between them, portions (A, A') thereof filling the holes and being clamped in place by the two magnetic core parts. The housing is enclosed in double outer shells to reduce the temperature sensitivity.
Another prior-art YIG component has a construction in which the magnetic core consists of a cylinder having a bottom, a cap and a central pin or pole pin, extending upwards from the bottom towards the cap and leaving a slot (pole gap) between the upper end of the pin and the cap. A coil is disposed around the pin. The remaining components are mainly arranged in the space defined between the magnetic coil and the cap of the magnetic core and are attached to the cap or the cylinder wall.
The above prior-art constructions have several drawbacks. Above all, they are relatively big, heavy and expensive because the magnetic material is a specific and expensive alloy which is difficult to work. The construction has been gradually minimized but size minimization is limited by the fact that the components which are accommodated therein require a fixed amount of space and by the fact that the resonator must be oriented to the center of the mechanical structure.
The thermal conductivity of magnetic iron is low and this is a disadvantage of the prior-art construction because a relatively high power dissipation from said coil and circuits must be cooled via this material.
Certainly, the prior-art YIG components may be controlled electrically but high inductance in the control coil and troublesome eddy currents have the consequence that changes of frequency are relatively time-consuming, thereby limiting the range of possible applications. The magnetic flux generated by the electromagnet flows through the pole pin (or pins) and through the walls of the housing, which is cylindrical or box-shaped. The magnetic flux thus passes through many parts of different sections and circumferences. When making a current change in order to change the resonance frequency, a flux change results. In that case, eddy currents are induced at each section/ circumference with a varying strength and decay time or time constant dependent on the section/circumference. These eddy currents initiate an exponential delay between tuning current and magnetization (frequency change). This delay may be compensated by a "driver", an electronic circuit for voltage-to-current transformation which is used for enabling the YIG component voltage to be controlled. A magnet of this conventional design initiates about five different time constants, which must be compensated for by an equal number of compensation networks, each of which must be defined in respect of proportionality and time constant in order to counteract said delay effectively.
The conventional magnet design generates a large leakage flux. The optimal situation is when the total magnetic flux passes through the pole gap or air gap between the pole pin and the cap, or pole pins (Uehara), respectively, but in the prior-art construction a significant part deflects away from the pole pin and passes to the walls of the housing outside the pole gap, generating excessive inductance.
Furthermore the conventional YIG components are sensitive to mechanical influences as well as external magnetic fields from fans, motors, etc., which may modulate the resonance frequency. Accordingly, a specific mechanical mounting and an external, magnetic shield of .mu.-metal arranged around the YIG component, respectively, are often required.
The YIG component is ordinarily used in a microwave system in which a number of electric functions are desirable, and in which the YIG component is intended for cooperation with other YIG components or other units. It follows from this that said components and units must be interconnected by means of external contacts, cables and mechanical devices.
Up to now, the range of application of the YIG components has been limited by the abovementioned drawbacks.